In a plasma processing apparatus, a target object is micro processed by plasma. For example, in a plasma etching processing apparatus shown in FIG. 9, an upper electrode 905 and a lower electrode 910 are arranged to be opposite to each other in a processing vessel 900 to generate plasma between the electrodes 905 and 910. The upper electrode 905 includes a gas shower head 905a and a quartz shield 905b, and a deposit shield 915 surrounding the quartz shield 905b. 
A baffle plate 925 is arranged at the periphery of a susceptor 920, which is connected to a high frequency power supply 935 via a matching unit 930. A gas supplied from a gas supply source 940 is introduced to the inside of the processing vessel from the shower header 905a and converted into plasma by high frequency electric field energy applied from the high frequency power supply 935 and an etching process is performed on a wafer W by using the plasma.
An exhaust port 945 is connected to an automatic pressure control (“APC”) valve 950 which is in turn connected to a turbo molecular pump (“TMP”) 955. A roughing vacuum pump 960 (hereinafter, referred to as “DRY”) 960 is arranged at the rear side of the TMP 955. The inside of the processing vessel 900 is roughly exhausted by the DRY 960, and vacuum exhausted by the TMP 955 with the opening degree of a body of the APC 950 controlled. During the process, the inside of the processing vessel is vacuum exhausted to maintain airtightness.
Hereinafter, a part of the processing vessel, which defines a wafer processing space U1 that is partitioned by the baffle plate 925 and arranged over the susceptor 920, is referred to as a processing chamber PC and a part of the processing vessel, which defines an exhaust space U2 that is a space between the baffle plate 925 and the pumps, is referred to as an exhaust chamber EC.
In general, the inside of the processing vessel is made of aluminum and its surface is anodic oxidized (alumite treated) to enhance plasma resistance. For example, an upper wall surface PC1 and a side wall surface PC2 of the processing chamber PC shown in FIG. 9 are alumite treated. Further, the gas shower head 905a of the upper electrode 905 has an aluminum surface which has been ceramic sprayed or alumite treated. The deposit shield 915 and the baffle plate 925 have been also ceramic sprayed or alumite treated. Similarly, a wall surface EC1 of the exhaust chamber EC has been also alumite treated. A part of the exhaust chamber EC, which includes the alumite material, is greater in area than that of the processing chamber PC, which includes the alumite material.
A reaction product created during plasma processing is attached and deposited to the inner wall of the processing chamber. A deposit is peeled off and dropped from the inner wall if it is accumulated with more than a prescribed thickness. The detached deposit becomes a contaminant that may deteriorate the production yield in processing a wafer. For this reason, maintenance work, such as exchanging parts included in the deposit shield, is usually done at a predetermined time. However, at that moment, the inside of the apparatus is exposed to the atmosphere. After maintenance work, the inside of the processing vessel exposed to the atmosphere is exhausted until a desired vacuum level is achieved. A technique of shortening time taken to perform vacuum exhaust is disclosed, for example, in Japanese Patent Application Publication No. 1995-161643 and Japanese Patent Application Publication No. 1994-31154.
In the prior art, however, the entirety of the inside of the apparatus is exposed to the atmosphere during maintenance work so that the exhaust chamber EC which is not necessarily opened during maintenance as well as the processing chamber PC that performs maintenance is exposed to the atmosphere. Meanwhile, a part of the exhaust chamber EC, which includes the alumite material, is greater in area than a part of the processing chamber PC, which includes the alumite material.
According to a relationship between vacuum exhaust time and gas release ratio per unit area at room temperature as shown in FIG. 10, among alumite, sprayed ceramics, and quartz, alumite has a greater gas release amount per unit area (gas release ratio) on the order of one or two digit in comparison with the other materials when the inside of the processing vessel 100 is vacuum exhausted. Here, the main component of the released gas is moisture.
Further, alumite has a greater moisture adsorbing amount through its film surface when exposed from vacuum to the atmosphere in comparison with the other materials. FIG. 11 depicts a relationship between vacuum exhaust time and moisture adsorbing amount per unit area in a case where the processing vessel has been alumite treated, and FIG. 12 depicts a relationship between vacuum exhaust time and moisture adsorbing amount per unit area in a case where sprayed ceramics has been used for the processing vessel. Referring to FIGS. 11 and 12, alumite has a greater moisture adsorbing amount per unit area on the order of about one digit in comparison with sprayed ceramics. Over time after initiation of exposure to the atmosphere, the moisture adsorbing amount increases and it requires a long time for the moisture adsorbing amount through the film surface to be saturated and converged. The moisture adsorbing amount through the film surface is a cause of releasing gas.
Meanwhile, as described above, area of the alumite included in the exhaust chamber EC is larger than area of the alumite included in the processing chamber PC. As a consequence, the alumite included in the exhaust chamber EC, which is not necessary for exposure to the atmosphere, absorbs a considerable amount of moisture in the atmosphere, and thus it requires a long exhaust time to obtain a good vacuum exhaust property inside the processing vessel after maintenance.
FIG. 13 is a graph illustrating an example of vacuum exhaust performance after assembling the apparatus. “Chamber arrival pressure” or “build-up property (leak rate)” is an index that indicates whether a vacuum exhaust property inside the processing vessel is satisfactory. Referring to FIG. 13A, assuming a proper chamber arrival pressure is 5.0×10−2 (mTorr) or less, it can be seen that eight or more hours of exhaust time are necessary. Referring to FIG. 13B, assuming that a proper build-up property (leak rate) is 1.0 (mTorr/min) or less, it can be seen that seven or more hours of exhaust time are necessary. From above, eight or more hours of exhaust time was required to satisfy both conditions, and this was one of primary causes of deteriorating productivity of the apparatus.
Moreover, the exhaust chamber EC does not include a heating means, such as a heater provided in the processing chamber PC or flow rate control valve. Accordingly, there is no means to actively control the temperature inside the exhaust chamber EC to suppress moisture in the atmosphere from being introduced into the chamber EC. Further, there is no means to accelerate gas release during vacuum exhaust, either.